Method for electrolysis of aqueous solutions of hydrogen chloride

ABSTRACT

A method for the electrolysis of aqueous solutions of hydrogen chloride in order to produce chlorine, characterized in that the following process parameters are maintained for initial operation:
         the anode half-element is filled with a 5 to 20% strength by weight hydrochloric acid,   the concentration of the hydrochloric acid is more than 5% by weight during initial operation,   the volumetric flow of the hydrochloric acid through the anode half-element is set in such a way that, at the start of electrolysis, the velocity of the hydrochloric acid in the anode space is from 0.05 cm/s to 0.15 cm/s,   the electrolysis is started with a current density of 0.5 to 2 kA/m 2 , and the current density is then increased continuously or discontinuously until the desired current density is reached.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is 35 U.S.C. 371 National Stage Application ofInternational Application No. PCT/EP02/11560 filed Oct. 16, 2002 whichclaims priority to German Application No. 10152275.4 filed Oct. 23,2001.

BACKGROUND OF THE INVENTION

The invention relates to a method for the electrolysis of aqueoussolutions of hydrogen chloride in order to produce chlorine by means ofgas diffusion electrode while maintaining defined operating parameters.

Aqueous solutions of hydrogen chloride, referred to below ashydrochloric acid, are formed as a waste product in many processes inwhich organic hydrocarbon compounds are chlorinated in oxidizing fashionwith chlorine. The recovery of chlorine from these hydrochloric acids isof economic interest. The recovery can be carried out electrolyticallyusing gas diffusion electrodes which consume oxygen in the cathode space(oxygen-consuming cathode).

A corresponding method is known from U.S. Pat. No. 5,770,035. Accordingto that document, the electrolysis takes place in an electrolysis cellhaving an anode space with a suitable anode, e.g. a titanium electrodewhich is doped or coated with precious metal and is filled with theaqueous solution of hydrogen chloride. The chlorine formed at the anodeescapes from the anode space and is fed for suitable treatment. Theanode space is separated from a cathode space by a commerciallyavailable cation exchange membrane. A gas diffusion electrode ispositioned on the cation exchange membrane on the cathode side. Acurrent distributor is located behind the gas diffusion electrode. Anoxygen-containing gas or pure oxygen is usually introduced into thecathode space.

The nature of the initial operation and normal operation of anelectrolysis cell has an influence on the service life of the anodes orof the anode half-element and therefore on the economic viability of themethod.

According to U.S. Pat. No. 5,770,035, therefore, an oxidizing agent, forexample iron(III) or copper(II) is necessarily added to the solutionwhich is to be electrolyzed in order to protect against corrosion. Theseadditives then have to be removed again from the hydrochloric acid bymeans of additional outlay of apparatus. Moreover, they contaminate thehydrochloric acid and may under certain circumstances have an adverseeffect on the action of the ion exchange membrane or lead tocrystallization. U.S. Pat. No. 5,770,035 does not disclose anyconditions for initial operation of the cell.

According to conventional methods for initial operation and normaloperation, considerable corrosion to the anode coating and to the anodemetal, for example titanium, beneath the coating of the anode isinevitable. The anode space, which consists of titanium, is also at riskfrom corrosion. Corrosion entails high operating costs, a high level ofoutlay on maintenance and environmental and recycling problems.

SUMMARY OF THE INVENTION

It was an object of the present invention to provide a method for theelectrolysis of aqueous solutions of hydrogen chloride with optimizedoperating parameters.

According to the invention, the object is achieved by the features ofclaim 1.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The subject matter of the invention is a method for the electrolysis ofaqueous solutions of hydrogen chloride in order to produce chlorine, inwhich method the following process parameters are maintained for initialoperation:

-   -   the anode half-element is filled with a 5 to 20% strength by        weight hydrochloric acid,    -   the concentration of the hydrochloric acid is more than 5% by        weight during initial operation,    -   the volumetric flow of the hydrochloric acid through the anode        half-element is set in such a way that, at the start of        electrolysis, the velocity of the hydrochloric acid in the anode        space is from 0.05 cm/s to 0.15 cm/s,    -   the electrolysis is started with a current density of 0.5 to 2        kA/m², and the current density is then increased continuously or        discontinuously until the desired current density is reached.

The optimum hydrochloric acid concentration for start-up, for initialoperation and for normal operation is approximately 13% by weight. Below5% by weight, the voltage rises, which can lead to the formation ofanodic oxygen. The voltage also rises above a concentration of 20% byweight, and the corrosion increases. In this case, the anode coating maybe damaged, for example, by a 25% by weight strength hydrochloric acidat 80° C. Therefore, for initial operation too, the hydrochloric acidconcentration has to be at least 5% by weight. In the context of thepresent invention, the term initial operation is to be understood asmeaning the operating time from the start of electrolysis until thedesired current density is reached.

The anode used is preferably a titanium electrode which is doped orcoated with precious metal. Chlorine serves to protect the anode metaland the metal which forms the anode space, e.g. titanium, fromcorrosion. Hydrochloric acid which has penetrated through micropores inthe anode coating can attack the anode metal, for example titanium. Inthe event of ongoing corrosion of the anode metal, the coating may flakeoff. Therefore, during initial operation, when the installation is idleand when it is being filled, it should be ensured that sufficientchlorine, or at least 1 mg/l, preferably at least 50 mg/l, particularlypreferably 300 mg/l, of free chlorine is present in the hydrochloricacid. In normal operation, after the desired current density has beenreached, this condition is virtually always fulfilled.

After the electrolysis cell has been assembled and the anode space hasbeen filled with hydrochloric acid, the hydrochloric acid is pumpedthrough the anode half-element and circulated. In the process, theelectrolysis cell has to be operated with a volumetric flow in the rangefrom 0.05 cm/s to 0.15 cm/s, in order to obtain an optimum efficiency ofthe electrolysis. In particular, correct normal operation cannot beachieved with a lower volumetric flow. The temperature of thehydrochloric acid is in this case initially preferably between 30 and50° C., and during normal electrolysis operation is in the range from 50to 70° C.

According to the invention, initial operation of the electrolysis celluses a current density of 0.5 to 2 kA/m², preferably 1 to 2 kA/m², veryparticularly preferably 1.5 kA/m², but a lower current density than thedesired current density which is subsequently to be reached. Starting upusing the desired current density ultimately causes the membrane to bedestroyed, since the heat which is evolved cannot be dissipatedsufficiently quickly. The desired current density should be over 1kA/m², but preferably in the range from 2 to 8 kA/m². The precise valuedepends on the quantity of chlorine to be produced. A desired currentdensity which is too low leads to insufficient chlorine gas beingevolved. This can lead to the electrolyte, which is discharged from theanode space via a standpipe, to be sucked back into the anode space outof the standpipe on account of the gas pressure being too low. To avoidthis, a foreign gas or chlorine would have to be added if insufficientchlorine were evolved.

The increase in the current density up to the desired current densityshould take place by no less than 0.5 kA/m² within 25 minutes but by nomore than 1.5 kA/m² within 5 minutes. Faster start-up, i.e. a fasterincrease in the current density from initial operation to the desiredcurrent density can cause the electrolysis cell to overheat, whichendangers the mechanical and chemical stability of the titanium.Furthermore, in the event of rapid start-up, the electrolyte can besucked back out of the standpipe into the anode space.

The increase may in this case preferably take place discontinuously, inwhich case it is particularly preferable for the current density to beincreased by in each case 0.5 to 1.5 kA/m², preferably by 1 kA/m², atintervals of from 5 to 25 min. Alternatively, however, the currentdensity can also be increased continuously until the desired currentdensity is reached.

In a preferred embodiment, the pressure difference between anode spaceand cathode space during initial operation until the desired currentdensity is reached is greater than 50 mbar, then in normal operation ispreferably greater than 100 mbar. This avoids additional transferresistances and a higher electrolysis voltage, which occur if thepressure is too low, since the gas diffusion electrode has to be pressedonto the cathodic current collector by the higher pressure in the anodespace. In normal operation, the anolyte is more compressible on accountof its chlorine content, and the density of the anolyte decreases as aresult of a rising chlorine content. Therefore, the pressure differencebetween anode space and cathode space in normal operation after thedesired current density has been reached is preferably greater than 100mbar.

After the desired current density has been reached, the volumetric flowof the hydrochloric acid can preferably be set in such a way that thevelocity of the hydrochloric acid in the anode half-element is from 0.2cm/s to 0.4 cm/s. This avoids siphoning-off via the standpipes and anuneven supply of liquid to the half-elements.

The method according to the invention can be optimized by thetemperature difference between the inlet for the hydrochloric acid intothe anode half-element (anolyte inlet) and the outlet for thehydrochloric acid from the anode half-element (anolyte outlet) beingless than 15° C. This allows a uniform, low temperature distribution inthe anolyte, which in particular avoids temperature peaks of over 60° C.

The method according to the invention is preferably to be used if theelectrolysis cell employed is an electrolyzer in which the electrolyteand the chlorine formed are discharged from the anode half-element via astandpipe.

The electrolyzer for carrying out the method according to the inventionusually comprises a plurality of electrochemical cells, in which caseanode and cathode half-elements are arranged alternately. The anodehalf-element is formed by the anode space and the anode, and the cathodehalf-element is formed by the cathode space and the gas diffusionelectrode as well as a current distributor. The anode and cathodehalf-elements are separated by a cation exchange membrane. In this case,the anode frame for forming the anode half-element, the cathode framefor forming the cathode half-element and the anode consist of stablematerials, such as for example titanium alloys or titanium doped orcoated with precious metal. The cation exchange membrane used can becommercially available membranes, such as for example the membraneNafion® 324 produced by DuPont. Oxygen or an oxygen-rich gas isintroduced into the cathode space. The method according to the inventioncan be carried out using commercially available gas diffusionelectrodes, e.g. produced by E-TEK (USA), with 30% of platinum onVulcan® XC-72 (activated carbon), with a precious metal coating on theelectrode of 1.2 mg of Pt/cm². The gas diffusion electrode is pressedonto the current distributor by the cation exchange membrane, asdescribed in EP-A 785 294, on account of a higher pressure in the anodespace than the cathode space. This produces sufficient electricalcontact.

EXAMPLES

The examples described below were carried out using an electrolysis cellcomprising an anode half-cell and a cathode half-cell. The anode usedconsisted of expanded titanium metal which had been activated with aruthenium oxide layer. A cation exchange membrane produced by DuPont,type Nafion® 324, was used to separate the anode space and the cathodespace. The cathode used was a carbon-based gas diffusion electrode witha precious metal coating produced by E-TEK (USA). The gas diffusionelectrode was connected to a current collector. The current collectorlikewise consisted of activated titanium expanded metal.

Example 1 (Hydrochloric Acid with Chlorine; in Terms of the HClConcentration, Serves as a Comparison for Example 2, and in Terms of theChlorine Content Serves as a Comparison for Comparative Example 1 andExample 3)

The electrolysis cell was filled with 9% strength by weight hydrochloricacid which contained 780 mg/l of free chlorine. Then, the supply ofoxygen to the cathode half-element was opened and the oxygen wassupplied at a volumetric flow of 1.25 m³/h.

The volumetric flow of the hydrochloric acid was set in such a way thatthe velocity of the hydrochloric acid at the start of electrolysis was0.1 cm/s. At the start of electrolysis, the current density was 1 kA/m²,and this current density was increased by in each case 1 kA/m³ atintervals of 15 minutes until the desired value for the current density(desired current density) of 4 kA/m³ had been reached. After the desiredcurrent density had been reached, the volumetric flow of thehydrochloric acid was increased in such a way that its velocity was 0.3cm/s. During initial operation, the hydrochloric acid concentration didnot at any time drop below 5% by weight. During normal operation of theelectrolysis cell, the hydrochloric acid concentration of 9% by weightwas maintained as a result of fresh concentrated hydrochloric acid (32%strength by weight) being supplied continuously while dilutehydrochloric acid and chlorine were discharged continuously. Thetemperature of the hydrochloric acid was 40° C. at the start (at 1kA/m²) and was increased to 60° C. When 3 kA/m² was reached, the anolytefeed no longer had to be heated, since the anolyte outlet temperaturewas approximately 60° C. At over 3 kA/m³, the anolyte feed was cooled,in order to ensure that the temperature of the anolyte discharge did notrise above 60° C. The temperature difference between the inlet andoutlet for the hydrochloric acid was at all times less than 15° C. Theelectrolysis voltage was 1.5 V at a desired current density of 4 kA/m².At the end of the test, no traces of corrosion could be observed at theanode and anode half-element.

Comparative Example 1 (Hydrochloric Acid without Chlorine; Corrosion)

The electrolysis cell was filled with 13% strength by weighthydrochloric acid which did not contain any chlorine. Then, the oxygensupplied to the cathode half-element was opened and the oxygen wassupplied with a volumetric flow of 1.25 m³/h. The volumetric flow of thehydrochloric acid was set in such a way that the hydrochloric acidvelocity at the start of electrolysis was 0.1 cm/s. At the start ofelectrolysis, the current density was 1 kA/m², and this current densitywas increased by in each case 1 kA/m² at intervals of 15 minutes untilthe desired value for the current density (desired current density) of 4kA/m² was reached. After the desired current density had been reached,the volumetric flow of the hydrochloric acid was increased in such a waythat the velocity was 0.3 cm/s. During initial operation, theconcentration of the hydrochloric acid did not at any point drop below5% by weight. During normal operation of the electrolysis cell, thehydrochloric acid concentration of 13% by weight was maintained by freshconcentrated hydrochloric acid (32% strength by weight) being suppliedcontinuously while dilute hydrochloric acid and chlorine were dischargedcontinuously. The temperature of the hydrochloric acid was initially 40°C. (at 1 kA/m²) and was increased to 60° C. The temperature differencebetween the inlet and outlet for the hydrochloric acid was at all timesless than 15° C. The electrolysis voltage when the desired currentdensity was reached was 1.43 V. At the end of the test, it was possibleto observe traces of corrosion at anode and anode half-element.

Example 2 (Influence of the HCl Concentration on Voltage when theDesired Current Density is Reached; a Voltage Minimum Lies at 13% byWeight)

The electrolysis cell was filled with 17% strength hydrochloric acidwhich contained 1280 mg/l of free chlorine. Then, the oxygen supply tothe cathode half-element was opened and the oxygen was supplied at avolumetric flow of 1.25 m³/h. The volumetric flow of the hydrochloricacid was set in such a way that the velocity of the hydrochloric acid atthe start of electrolysis was 0.1 cm/s. At the start of electrolysis,the current density was 1 kA/m² and this density was increased by ineach case 1 kA/m² at intervals of 15 minutes until the desired value forthe current density (desired current density) of 4 kA/m² was reached.After the desired current density had been reached, the volumetric flowof the hydrochloric acid was increased in such a way that its velocitywas 0.3 cm/s. During initial operation, the concentration of thehydrochloric acid did not drop below 5% by weight at any time. Duringnormal operation of the electrolysis cell, the hydrochloric acidconcentration of 17% by weight was maintained by fresh concentratedhydrochloric acid (32% by weight) being supplied continuously whiledilute hydrochloric acid and chlorine were discharged continuously. Thetemperature of the hydrochloric acid was initially 40° C. (at 1 kA/m²)and was increased to 60° C. The electrolysis voltage was 1.47 V at adesired current density of 4 kA/m². At the end of the test, no traces ofcorrosion could be observed at the anode and anode half-element.

Example 3 (Hydrochloric Acid with Chlorine Content; No Corrosion)

The procedure was as in Comparative Example 1, except that thehydrochloric acid was additionally mixed with chlorine. The electrolysiscell was filled with 13% strength by weight hydrochloric acid whichcontained 200 mg/l of free chlorine.

Then, the oxygen supply to the cathode half-element was opened and theoxygen was supplied at a volumetric flow of 1.25 m³/h. The volumetricflow of the hydrochloric acid was set in such a way that the velocity ofthe hydrochloric acid at the start of electrolysis was 0.1 cm/s. At thestart of electrolysis, the current density was 1 kA/m², and this densitywas increased by in each case 1 kA/m² at intervals of 15 minutes untilthe desired value for the current density (desired current density) of 4kA/m² had been reached. After the desired current density had beenreached, the volumetric flow of the hydrochloric acid was increased insuch a way that the velocity was 0.3 cm/s. During initial operation, theconcentration of the hydrochloric acid did not drop below 5% by weightat any time. During normal operation of the electrolysis cell, thehydrochloric acid concentration of 13% by weight was maintained by freshconcentrated hydrochloric acid (32% strength by weight) being suppliedcontinuously while dilute hydrochloric acid and chlorine were dischargedcontinuously. The temperature of the hydrochloric acid was initially 40°C. (at 1 kA/m²) and was increased to 60° C. The temperature differencebetween inlet and outlet for the hydrochloric acid was less than 15° C.at any time. The electrolysis voltage was 1.43 V at a desired currentdensity of 4 kA/m². No traces of corrosion in the anode half-elementwere observed even after an operating time of 2400 h.

Example 4 (Influence of the Hydrochloric Acid Flow Velocity)

The electrolysis cell was filled with 13% strength by weighthydrochloric acid which contained 200 mg/l of free chlorine. Then, theoxygen supply to the cathode half-element was opened and the oxygen wassupplied at a volumetric flow of 1.25 m³/h. The volumetric flow of thehydrochloric acid was set in such a way that the velocity of thehydrochloric acid at the start of electrolysis was 0.2 cm/s. Thetemperature of the hydrochloric acid was set to 40° C. Initial operationcould not commence, since strong pressure pulses were formed, which ledto safety cut-outs. The safety cut-out is intended to prevent damage inparticular to the cation exchange membrane and the gas diffusionelectrode and also to the electrolysis half-elements as a whole. It wasonly possible to start electrolysis when the flow velocity was reducedto 0.14 cm/s.

The current density was 1 kA/m² at the start of electrolysis and wasincreased by in each case 1 kA/m² at intervals of 15 minutes until thedesired value for the current density (desired current density) of 4kA/m² was reached. After the desired current density had been reached,the flow velocity for long-term operation was increased to 0.3 cm/s.During initial operation, the hydrochloric acid concentration did notdrop below 5% by weight at any time. During normal operation of theelectrolysis cell, the hydrochloric acid concentration of 13% by weightwas maintained by fresh concentrated hydrochloric acid (32% strength byweight) being supplied continuously while dilute hydrochloric acid andchlorine were discharged continuously. The temperature of thehydrochloric acid was increased from initially 40° C. (at 1 kA/m²) to60° C. The temperature difference between inlet and outlet for thehydrochloric acid was less than 15° C. at all times. The electrolysisvoltage was 1.43 V at the desired current density.

1. A method for the electrolysis of aqueous solutions of hydrogenchloride in order to produce chlorine, wherein the following processparameters are maintained for initial operation: the anode half-elementis filled with a 5 to 20% strength by weight hydrochloric acid thatcontains at least 1 mg/l of free chlorine, the concentration of thehydrochloric acid is at least 5% by weight during initial operation, thevolumetric flow of the hydrochloric acid through the anode half-elementis set in such a way that, at the start of electrolysis, the velocity ofthe hydrochloric acid in the anode space is from 0.05 cm/s to 0.15 cm/s,the electrolysis is started with a current density of 0.5 to 2 kA/m²,and the current density is then increased continuously ordiscontinuously until the desired current density is reached.
 2. Themethod as claimed in claim 1, wherein, during normal operation, theconcentration of the hydrochloric acid in the anode half-element is setin the range from 5 to 20% by weight.
 3. The method as claimed in claim1, wherein the current density is increased by in each case 0.5 to 1.5kA/m² at intervals of from 5 to 25 min.
 4. The method as claimed inclaim 1, wherein, after the desired current density has been reached,the volumetric flow of the hydrochloric acid is set in such a way thatthe flow velocity of the hydrochloric acid in the anode half-element isfrom 0.2 cm/s to 0.4 cm/s.
 5. The method as claimed in claim 1, whereinthe desired current density is greater than 1 kA/m².
 6. The method asclaimed in claim 1, wherein the pressure difference between anode spaceand cathode space during initial operation until the desired currentdensity is reached is greater than 50 mbar.
 7. The method as claimed inclaim 1, wherein the pressure difference between anode space and cathodespace after the desired current density has been reached is greater than100 mbar.